Disks provide the bulk of secondary storage for modern computer systems megnetic tap was used as an early secondary storage medium although it is relatively permanent and can hold large no. of data magnetic tap is slow in coparison to the access time of main memory even more importent meg netic tap is limited to sequential access thus it is unsuitable for providing the random access needed by files taps are curantaly used mainly for backup for storage inferquently used information and a medium for transferring information from one system to another

Information on the disk is referenced by a multipart address wihich inincludes the driver no. the surface, the track, and sectore. all the tracks on one drive that can be accessed without the heads being move (the equivalent tracks on the different surfaces) consti tues a sylender within a track information is stored in sectores. a sector is the smalest unit of information that can be read from or written to the disk.

I/O transfers betbeen memory and disk are performed in units of one or more sectors called blocks we can convert from a disk add. of sylinder i, surface j, sectors k to a one dimentional block no. be by b=k+s *(j+i*t).